7 years ago, I had a tkr done on my left knee.

2 years ago I got a bone infection and had to have a revision.

In march, the plantaris tendon behind the knee ruptured.

Now I am developing a big knot behind the knee.

Should I be concerned? It is difficult to walk.

Do I need to see a specalist, and if so what kind?

You do need to see a specialist but I couldn't tell you off the top of my head. I would go to your family doctor/general physician and talk to them about it to get the referral. Given your history it could be something serious.